Modular and low power ionizer

ABSTRACT

An apparatus for ionizing air to remove particulate matter. The apparatus including an ionizer and a bracket for mounting the ionizer and a bracket for mounting the ionizer in a duct or enclosure. The ionizer includes a series of electrodes which span a portion of the duct. The electrodes are energized by a high voltage circuit and an ionic wind is created between the electrode and duct. The ionic wind sweeps particles in the air to the duct which provides a collector electrode. In another embodiment, a ring collector electrode is also provided for spanning the inner portion of the duct. The high voltage circuit includes a DC power supply, a high voltage transformer, a high voltage multiplier stage and a push-pull switching circuit. The DC power supply receives AC power and generates a DC output which is coupled to the primary of the transformer. The push-pull switching circuit produces a controlled and efficient AC output in the transformer by alternately switching the primary winding. The output voltage from the secondary winding is further increased by the multiplier stage to a level sufficient to energize the electrodes and produce the ionic wind. The apparatus also comprises a high voltage multiplier stage.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/457,680, filed on Jun. 1, 1995 now U.S. Pat. No. 5,578,112,and a national stage application of International Patent Application No.PCT/CA96/00358.

This invention relates to ionizers and more particularly to a modularand low power ionizer suitable for commercial and residential use.

BRIEF SUMMARY OF THE INVENTION

Conventional ionizers or precipitators comprise large and veryspecialized devices. These devices are intended for large industrialapplications, for example a cement factory, and have high powerrequirements. Due to their large power requirements, the ionizersinclude separate high voltage power supplies and tend to be very bulkyand costly to manufacture and maintain. The devices are typicallydesigned as stand-alone units which are coupled to existing ventilationor heating and cooling equipment. For these reasons, known devices arenot well-suited for commercial applications, such as once buildings, orresidential or consumer use. Published European Patent Application No.90850276.8 discloses one such device according to the prior art.

In an office building, the air circulation system includes a filter bankwhich comprises a matrix of filter modules. Each filter module typicallyhas a mechanical filter element which traps particulate matter in theair before the air is circulated. The filter elements need to bereplaced on a regular basis thereby incurring both maintenance andreplacement costs. There is also a cost associated with the disposal ofthe used filter elements. For medical facilities, the filter elementsare treated as hazardous biological waste and the disposal costs aresignificant. Furthermore, the air circulation fans must have thecapacity to push the "dirty air" through the filter elements. For atypical once building this means large electric motors with a highhorsepower output to drive the circulation fans, which further increasesthe cost of a conventional air conditioning/heating installation.

There is also reason to believe that filter elements which have becomecontaminated may contribute to "sick building syndrome".

BRIEF SUMMARY OF THE INVENTION

Accordingly, there is a need for an ionizer which is suitable forcommercial and residential use. It is an object of the present inventionto provide a modular ionizer which may be integrated with an existingheating or cooling duct in the heating and cooling equipment (HVAC) of abuilding. It is another object of the present invention to provide anionizer with an integrated high voltage generator which is operated fromconventional AC power and features low power consumption. It is afurther object of the present invention to provide an ionizer whichproduces negligible amounts of ozone as a by-product of the ionizationprocess. It is yet a further object of the present invention to providea modular ionizer which is arranged with other ionizer modules to forman ionizer bank or matrix suitable for use in larger installations suchas those found in residential condominiums, office buildings, medicalfacilities, laboratories, food processing plants, electronic assembly(i.e. "clean-room") plants, and manufacturing and industrial plants.

In a first aspect, the present invention provides an apparatus forpurifying gas flowing in a duct by establishing a radially directedionic wind within the duct to sweep particulate solids directly onto oneor more collector electrodes, said apparatus comprising: (a) an ionizingunit; (b) means for supporting said ionizing unit within the duct, saidionizing unit comprising, (i) a water-tight housing, (ii) a high voltagegenerator within the housing and having a high voltage output, (iii) anelectrode support rod coupled to said high voltage output and extendingfrom said housing coaxially within said duct, (iv) at least one group ofionizing electrodes mounted on said support rod and extending radiallytherefrom; and (c) means for connecting said high voltage generator toan external low voltage power supply.

In a second aspect, the present invention provides an air purifier forpurifying air in an enclosed space and said enclosed space beingprovided with an AC power supply, said air purifier comprising: (a) anenclosure having at least one collecting electrode; (b) an ionizingunit; (c) means for supporting said ionizing unit inside said enclosure,said ionizing unit comprising, (i) a water-tight housing, (ii) a highvoltage generator within said housing for generating a high voltageoutput, (iii) an electrode support rod coupled to said high voltageoutput and extending from said housing coaxially within said duct, (iv)at least one group of ionizing electrodes mounted on said support rodand extending radially therefrom for establishing a radially directedionic wind within said enclosure to sweep particulate solids in the airdirectly onto said collector electrode; (d) means for connecting saidhigh voltage generator to the external AC power supply; and (e) saidenclosure including an air intake port and an air exhaust port.

In a third aspect, the present invention provides a high voltagemultiplier stage comprising: (a) an input port for receiving an inputvoltage signal; (b) a body member having two side channels for mountingcapacitors and a bottom channel for mounting diodes, and said capacitorsand diodes being coupled to form a plurality of stages for said highvoltage multiplier; (c) said bottom channel being disposed between saidside channels; (d) said last stage providing an output port for saidhigh voltage multiplier.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the drawings which illustrate, by way ofexample, a preferred embodiment of the present invention, and in which:

FIG. 1 is a cross-sectional view of an ionizing apparatus according tothe present invention;

FIG. 2 is a block diagram of circuitry of the apparatus of FIG. 1;

FIGS. 3(a) to 3(c) show the circuitry of FIG. 2 in schematic form;

FIG. 4(a) shows a bank of ionizers according to the invention;

FIG. 4(b) shows another arrangement for a bank of ionizers according tothe invention;

FIG. 5 is a timing diagram showing the relationship between selectedcontrol signals generated in the circuit of FIG. 3;

FIGS. 6(a) and 6(b) show in schematic form a transformer according tothe present invention; and

FIGS. 7(a) to 7(c) show in schematic form an embodiment for a highvoltage multiplier according to the present invention, and wherein FIG.7(a) is a top view of the high voltage multiplier, FIG. 7(b) is a sideview, and FIG. 7(c) is an end view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first made to FIG. 1 which shows an ionizing apparatus 1according to the present invention. The ionizing apparatus 1 comprises atubular member or conduit 2 and an ionizer 4. As shown, the ionizer 4 ismounted coaxially inside the tubular member 2 by a support bracket 6.The tubular member 2 can comprise an existing duct connected to theheating and cooling equipment (HVAC) of a building. Alternatively, thetubular member 2 can comprise a separate member which provides a housingor enclosure and an ionization chamber for the ionizer 4. The supportbracket 6 also provides a power feed for the ionizer 4. The power feedcomprises a power cable 8 coupled to a mains supply module 10.

In FIG. 1, the ionizer 1 is shown mounted horizontally in the tubularmember 2. It will be appreciated that the ionizer 1 may also be mountedvertically in a vertical tubular member for example.

It is a feature of the present invention that the ionizer 4 is poweredusing conventional AC or "mains" power and the supply module 10 issimply plugged into a wall socket. The module 10 includes a conventional115 VAC transformer. Furthermore, the modular nature of the device 1allows the ionizer 4 to be integrated with the existing heating orcooling system of a building. For example, the tubular member 2 cancomprise a heating duct connected to the furnace. The bracket 6 attachesthe ionizer 4 to the duct 2 and power is provided by an electricaloutlet. For larger applications, e.g. an once building, the ionizers 4are arranged in a bank or matrix 3 as shown in FIG. 4(a). The bank 3comprises a plurality of modules or cells 3a, 3b, 3c, 3d. . . eachhaving an ionizer 4 (as depicted for the first cell 3a). The bank ormatrix 3 of ionizers 4 can replace or augment the existing air filterbank (not shown). The ionizers 4 can also be "daisy chained" inside theair circulation duct as shown in FIG. 4(b). In FIG. 4(b), another columnof modules 5a, 5b, 5c, 5d are located behind the modules 3a, 3b, 3c, 3d.This arrangement can increase the amount of particulate matter removedfrom the air.

Alternatively, the device 1 is manufactured as a stand-alone unit whichis positioned in a room, for example in a residential home, and pluggedinto a wall socket. The stand-alone unit includes intake and exhaustports and can also have a fan (not shown). It will be appreciated thatsuch a stand-alone unit will need appropriate EMI shielding and safetyfeatures.

As shown in FIG. 1, the ionizer 4 comprises a water-tight enclosure 12which houses a high voltage circuit 14. One end of the enclosure 12 issealed by an aluminum lid 16 which also acts as a heat sink for the highvoltage circuit 14. The heat sink capability of the lid 16 is augmentedby the flow of air 34 through the duct 2, however, the direction for theair flow 34 can be opposite to that shown in FIG. 1. A cap 18 isattached to the other end of the enclosure 12 and provides a water-tightseal. Attached to the cap 18 through a sealed (e.g. rubber gasket)opening 119 is an electrical discharge rod 20. The discharge rod 20 iselectrically coupled through a contact 121 to the circuit 14 andreceives the high voltage output generated by the circuit 14. Theelectrical discharge rod 20 preferably includes two or more groups ofionizing electrodes with two groups of electrodes 22,24 being shown inFIG. 1. Each group of ionizing electrodes 22,24 comprises four wires26a, 26b, 26c and 28a, 28b, 28c with the fourth wire not being shown.The distance between adjacent ionizing electrodes, i.e. 26a and 28a, isapproximately 18 inches. Each group of ionizing electrodes 22,24 cancomprise more than four wires, but preferably there are at least fourwires. In another embodiment, the ionizing electrodes 22,24 may bereplaced by a wire mesh having openings of approximately 0.5 inchessquare.

As shown in FIG. 1, there is also provided a ring 30. The ring 30 iscoupled to the duct 2 by a bracket 32 as shown in FIG. 1. Alternatively,the ring 30 is supported by an insulated bracket 32' (shown in brokenoutline) which is connected to the rod 20 and the ring 30 is held at thedesired potential, e.g. ground, using an insulated wire 33. The ring 30is made from a conductive material such as copper and provides acollector electrode for the second group of ionizing electrodes 24. Asshown in FIG. 1, the ionizing electrodes 28a, 28b are shorter than theionizing electrodes 26a, 26b in the first group 22, and the combinationof the ring 30 and ionizing electrodes 28a, 28b (and 28c, 28d) ionizes aportion of the airflow in the duct 2 which does not pass over the firstgroup of ionizing electrodes 22. The ring 30 is suitable for use withthe high voltage circuit 14 for producing a 60 kV output.

In operation, the high voltage circuit 14 produces a high voltage outputfrom about 60 kV to 135 kV at 150 Watts. (The transformer, i.e. triplecore, arrangement shown in FIG. 6 is utilized for producing the 135 kVoutput.) The high voltage output energizes the discharge rod 20 and theionizing electrodes 26,28. A flow of "dirty" air (or gas) 34 is passedthrough the ionization chamber, e.g. duct 2, and the air molecules andparticulate matter in the air flow 34 are ionized as they pass by theionizing electrodes 26,28. (The dirty air 34 will contain particulatesolids, such as dust, smoke and the like.) An ionic wind 127 (shownusing a broken line outline) is produced between the wires 26 formingthe first ionizing electrode group 22 and the inside surface of the duct2. The duct 2 (i.e. inside surface of the duct 2) provides a collectorelectrode for collecting particles which are picked up by the action ofthe ionic wind 127. The primary function of the duct 2 is to act as a"collector" electrode to collect the particulate solids which are sweptby the ionic wind 127 created by the ionizing electrodes 26a, 26b, 26c,26d. (The duct 2 can also act as a "Faraday" cage or shield.) Similarly,for the second group of ionizing electrodes 24, an ionic wind 29 (shownusing a broken line outline) is generated between the tips of theionizing electrodes 28a, 28b, 28c, 28d and the ring 30. The ionic wind129 is predominantly negative, and therefore the ring 30 is grounded bythe duct 2 or the insulated wire 33. The ionic wind 129 generated by thesecond group of ionizing electrodes 24 is intended for dirty air 36which flows inside of the ionizing electrodes 26a, 26b, 26c, 26d, forexample, due to the turbulence caused around the end cap 18. Theresulting clean air flow 34' and 36' continues to pass through the duct2.

It has been found that the efficacy of the ionizer 4 increases with thearrangement of the ring 30 and the ionizing electrodes 28a, 28b, 28c,28d. Preferably, the distance between tips of the ionizing electrodes26a, 26b, 26c, 26d (or electrodes 28a, 28b, 28c, 28d) and the duct 2 (orring 30) is in the range 10 to 15 cm. It will be appreciated that thedistance is also dependent on the field strength produced by the ionizer4.

There may also be applications where is advantageous to replace the ring30 and electrodes 28 with electrodes (not shown) which extendapproximately the same distance as the electrodes 26. For example, whengenerating a full 135 kV output the ring 30 is not used.

As shown in FIG. 1, the ionizing apparatus 1 can also include a controlpanel 38. The control panel 38 provides a user interface for set-up andmaintenance. The control panel 38 includes an ON/OFF switch 40, a POWEROUTPUT indicator 42 and an output current meter 44. Optionally, thecontrol panel 38 may include a RESET switch 46.

For maintenance, a water jet is used to clean the particles which haveaccumulated on the inside surface of the duct 2 or on the surface of thering 30. Because the enclosure 12 is water-tight, the device 1 may beconveniently washed without removing the ionizer 4 from inside the duct2. Alternatively, the modular nature of ionizer 4 and support bracket 6permit the ionizer 4 to be removed and the inside surface of the duct 2(or housing) scrubbed or washed without the ionizer 4 in place.

As shown in FIG. 1, a feature of the present invention is thearrangement of the high voltage circuit 14 inside a water-tightenclosure 12 which is mounted coaxially in the member 2. Thisarrangement simplifies construction and installation of the device 1 inexisting duct work, and also facilitates cleaning of the device 1. Themodular nature of the ionizer 4 also makes it suitable for forming abank or matrix 3 as shown in FIGS. 4(a) and 4(b). Another feature of thepresent invention is the capability to use conventional AC power tooperate the device 1 which makes the device 1 attractive for wide-spreadapplications, e.g. commercial office buildings and private residentialhomes, in addition to industrial applications. This advantage isrealized by the high voltage circuit 14 according to this aspect of thepresent invention.

The high voltage circuit 14 is shown in block diagram form in FIG. 2.The high voltage circuit 14 comprises a pulse control module 50. Thepulse control module 50 is coupled to an output drive module 52. Theoutput drive module 52 comprises a "push-pull" circuit which drives theprimary winding of a high voltage transformer 54. As will be described,the pulse control module 50 produces pulse signals for controlling the"push-pull" circuit in the output drive module 52. The output, i.e.secondary winding, of the high voltage transformer 54 is coupled to ahigh voltage multiplier 56. The high voltage multiplier 56 increases thevoltage in the secondary winding of the transformer 54 to a suitablehigh voltage level at an output port 58. (The high voltage multiplier 56is described below with reference to FIG. 7) The high voltage output 58is coupled to the discharge rod 20 (FIG. 1) through an electricalcontact terminal 121 (FIG. 1). The high voltage transformer 54 togetherwith the high voltage multiplier 56 generate the high voltage output(e.g. up to 135 kV) for energizing the ionizing electrodes 26,28connected to the discharge rod 20 (FIG. 1).

According to another aspect of the present invention the high voltagetransformer 54 (depicted in FIG. 6) and the high voltage multiplier 56(depicted in FIG. 7) form a "tuned" circuit to operate the transformer54 in resonance to generate the high output voltage levels, i.e 135 kV.

Referring to FIG. 2, the high voltage circuit 14 includes an oscillator60. The oscillator 60 provides a reference trigger signal for the pulsecontrol module 50. The output from the oscillator 60 is coupled to thepulse control module 50 through a buffer stage 62. The buffer stage 62provides the drive for the reference trigger signal and prevents loadingof the output from the oscillator 60.

The high voltage circuit 14 also includes an output regulator 64 asshown in FIG. 2. The output regulator 64 is coupled to the transformer54 and the pulse control module 50. The output regulator 64 comprises afeedback circuit which controls the pulse control module 50 on the basisof the output of the high voltage transformer 54. As will be describedin more detail below, the output regulator 64 is configured to regulatethe output voltage or the output current. In current regulation mode,the output current is maintained at a predetermined value, e.g. 250 μA,and the voltage level is allowed to vary in a range, e.g. 60 kV to 135kV. In voltage regulation mode, the output voltage level is maintainedat a preselected value as set by the potentiometer 146 (FIG. 3(c)).

The high voltage circuit 14 is powered by the power supply module 70.The power supply module 70 comprises the mains supply module 10 (FIG. 1)which is connected to the mains supply through a conventional cable andplug. The mains supply module 10 provides a 140 VDC output at terminal11 and a 20 VAC output at terminal 13. The mains supply module 10 isshown in FIG. 3(c) and comprises an AC line transformer 15. The primarywinding of the AC line transformer 15 is coupled to the mains supplycable and includes a wired fuse 17 and the ON/OFF switch 40. As shownthe terminal 11 for 140 VDC supply is coupled to the primary of thetransformer 15 through a resistor 400 and rectified by a diode 401. Thesecondary winding "steps down" the voltage to provide the 20 VAC supplyat the terminal 13. The 140 VDC and 20 VAC terminals 11, 13 and aterminal 9 for ground (GND) are coupled to the high voltage circuit 14through a cable 8.

The cable 8 comprises a multi-conductor cable and additional conductorsare included for connecting the circuit 14 (located in the enclosure 12)to the lamp 46 through terminal 19, to a voltage output level adjustpotentiometer 146 through terminals 21, 23, to the current meter 48through terminals 25, 27. The cable 8 also includes a conductor whichconnects the lid 16 (FIG. 1) to a surge suppressor, i.e. ground spike,circuit 39 shown in FIG. 3(b) and comprising a varistor 41. Theconductor 29 is also coupled to the chassis for the control panel 38.

Referring back to FIG. 2, the high voltage circuit 14 also includes anoverload protection module 66 and short circuit protection module 77.The overload protection module 66 has an input coupled to the highvoltage multiplier 56, an input coupled to the short circuit protectionmodule 77, an output coupled to the buffer stage 62, and another outputcoupled to the output regulator 64. The input from the high voltagemultiplier 56 preferably includes a filter network 87 which is describedin more detail below.

If the output current of the high voltage circuit 14 exceeds apredetermined amount, the overload protection circuit 66 is triggered todisable the pulse control module 50 (through the buffer stage 62 input)and thereby the output drive module 52.

The overload protection module 66 is also triggered if a short circuitcondition exists, for example, the high voltage transformer 54malfunctions and draws a large current. If a short circuit conditionoccurs, the short circuit protection module triggers the overloadprotection module 66. The overload protection module 66, in turn, shutsdown the circuit 14 by disabling the pulse control module 50 and outputdrive module 52.

Once triggered the pulse control module 50 remains disabled until theoverload protection module 66 is reset. The high voltage circuitincludes a reset module 68 for resetting the overload protection module66. The reset module 68 automatically generates a reset signal after apredetermined time. The reset module 68 can also be activated bymanually depressing a reset switch (not shown) located on the controlpanel 38. If the overload protection module 66 is triggered by the shortcircuit module 77, then preferably the circuit 14 is enabled onlymanually so as to provide an opportunity to investigate the cause of theshort circuit.

The oscillator 60 also provides a timing reference signal for the resetmodule 68 in order to provide for the automatic reset feature. Thisfeature is described in more detail below.

Reference is next made to FIGS. 3(a) to 3(c) which show the high voltagecircuit 14 in more detail. The values for the components shown in FIG. 3are listed in the Table below. As shown in FIG. 3(a), terminal 72connects to terminal 11 for the 140 VDC input and terminal 74 connectsto terminal 13 for the 20 VAC input. The ground terminal 9 is connectedto terminal 76. The 140 VDC input is "smoothed" by a capacitor 402 andresistor 403. The 20 VAC input is rectified by a diode 404 and smoothedby a capacitor 405. The rectified 20 VAC provides the input to a voltageregulator 78 on terminal 80. The voltage regulator 78 is shown in FIG.3(b) and comprises a conventional device such as the LM7812 fromNational Semiconductor. The regulator 78 provides a +12 Volt rail 82 andincludes a capacitor 406 for smoothing the +12 Volt output. The +12 Voltrail 82 provides the supply voltage for components comprising the highvoltage circuit 14. The high voltage circuit 14 also includes the surgesuppressor 39. The function of the surge suppressor 39 is protect thecircuit 14 by shunting voltage spikes to ground. As shown the surgesuppressor 39 comprises a varistor 41 with one terminal connected toground and the other terminal connected to the conductor 29 which isalso connected to the chassis.

Referring back to FIG. 3(a), the 140 VDC feed energizes the primarywinding of the high voltage transformer 54. The primary winding of thetransformer 54 has a centre-tap 84 which receives the 140 VDC. As shownin FIG. 3(a), the 140 VDC is coupled to the centre-tap 84 through abranch of the short circuit protection module 77. (The terminals of theprimary winding are connected to the output drive module 52 and operatedin a "push-pull" manner as described below.)

The short circuit protection module 77 comprises an inductor 86connected in series between the 140 VDC input 72 and the centre-tap 84of the high voltage transformer 54. The branch also includes a wiredfuse 88 and a capacitor 407 in parallel with the inductor 86. Theinductor 86 is magnetically coupled to a reed switch 90. One terminal ofthe reed switch 90 is coupled to a voltage input 92. The other terminalof the switch 90 forms an output 94 which is coupled to an input of theoverload protection module 66. As shown in FIG. 3(a), the outputterminal of the reed switch 90 includes resistors 408, 409 and capacitor410 for conditioning the output signal at terminal 94.

If a short circuit condition arises, the high voltage transformer 54will draw current which produces a magnetic field in the inductor 86.The magnetic field, in turn, "trips" the reed switch 90 which produces apulse at terminal 94 for the overload protection module 66 at theterminal 94. The resulting pulse triggers the overload protection module66 and causes the pulse control module 50 to become disabled as will bedescribed in more detail below.

In a variation of the short circuit protection module 77 the Reed switch90 and inductor 86 are replaced by a photodiode and phototransistorarrangement as shown in FIG. 3(a) and denoted by reference 79. Theimplementation of the circuit 79 as shown in FIG. 3(a) is within theknowledge of one skilled in the art.

Reference is next made to FIG. 3(b). The oscillator 60 is implementedusing a programmable timer chip 96, such as the MC14541 available fromMotorola Corporation. The timer chip 96 is configurable to provide anoutput signal 98 on pin 1 between 16 kHz to 22 kHz. The appropriateselection of the values for resistors 411, 412 and capacitors 413, 414coupled to pins 2 and 3 of the timer chip 96 and the voltage levelconnected to pin 12 is within the understanding of those skilled in theart. (Exemplary values for the resistors and capacitors are provided inthe Table below for a 19 kHz signal 98.) The timer chip 96 also includesa 16-stage binary counter which provides a timing signal on output pin 8for use by the reset module 68.

The 19 kHz signal 98 provides the reference signal for the pulse controlmodule 50 and is buffered by the buffer stage 62. As shown in FIG. 3(b),the buffer stage 62 is implemented using a single package chipcontaining six inverters 100 to 110, such as the MC4049 available fromMotorola Corporation. The individual inverters are cascaded in pairs100,102 and 104,106 and 108,110 to produce respective non-invertingbuffers. The buffer stage produces two buffered output reference signals112, 114 which are 180° out of phase. The buffered output signals 112,114 provide the reference inputs to the pulse control module 50. Asshown in FIG. 3(b), the inverters 104, 108 are also coupled to theoverload protection module 66 through respective diodes 415, 416 andresistors 417, 418. This aspect is described in further detail below.

The pulse control module 50 and the output drive module 52 comprise a"push-pull" arrangement which drives the high voltage transformer 54.The push-pull arrangement produces a more efficient power output fromthe high voltage transformer 54.

As shown in FIG. 3(a), the output drive module 52 comprises a pair ofpower transistors 116, 118. The outputs, i.e. drain and source, of thetransistors 116, 118 are coupled to the primary winding of the highvoltage transformer 54. The control input, e.g. gate, of the transistors116, 118 are coupled to respective outputs 120, 122 of the pulse controlmodule 50. The pulse control module 50 produces pulse trains 124, 126which switch the respective transistors 116, 118 ON and OFF, therebycontrolling the current flowing in the primary winding of thetransformer 54. As described above, the high voltage transformer 54 hasthe centre-tap 84 on the primary winding which is connected to the 140VDC supply. The current flowing in the primary winding induces a voltagein the secondary winding of the transformer 54. The voltage induced inthe secondary winding is multiplied by the high voltage multiplier 56 togenerate a high voltage at the output 58.

The pulse control module 50 comprises a pair of monostablemultivibrators 128, 130 which are implemented using first and secondLM555 type timer chips 132 and 134. As will be understood by thoseskilled in the art each of the 555 timer chips 132, 134 has a network ofresistors and capacitors which configure the timer chips 132, 134 asmonostable multivibrators (i.e. pulse generators). (Exemplary values forthe resistors 419 to 425 and capacitors 426, 427 are provided in theTable below.) The reference signal 112 provides the "trigger" signal forthe first monostable vibrator or pulse generator 128, and the referencesignal 114 provides the "trigger" signal for the second monostable 130.In response to the reference signal 112, the first pulse generator 128generates the pulse signal output 124 which drives the gate of the powertransistor 116. Similarly, the second pulse generator 130 produces thepulse signal 126 which drives the gate of the second power transistor118. The duty cycle of each pulse signal 124, 126 is determined by aresistor/capacitor network connected to the THRESHOLD and DISCHARGEinputs of the respective 555 timer chip 132, 134 as will be within theunderstanding of those skilled in the art. In the preferred embodiment,the duty cycle is approximately 25%. To protect the transistors 116, 118the outputs of the pulse generators 128, 130 include resistors 428, 429as shown in FIG. 3(b).

Preferably, the transistors 116, 118 comprise insulated-gate bipolarpower transistors of the type available from International Rectifier,e.g. model no. IRGPC50FD2 is suitable.

The relationship between the pulse signals 124, 126 is shown in FIG. 5.There is a phase shift or time lag between the pulse signals 124, 126which produces the "push-pull" action for the power transistors 116,118, i.e. when the first transistor 116 is ON, the second transistor 118is OFF. When the high voltage circuit 14 is set to full power output(e.g. using potentiometer 146--FIG. 3(c)), each pulse has a width ofapproximately 15 microseconds. At minimum power output, the pulse widthfor the pulse signals 124, 126 is approximately in micro-second range.

Referring back to FIG. 3(a), the primary winding of the high voltagetransformer 54 includes the centre-tap 84 which is connected to the 140VDC feed from the power supply module 70. In response to the pulsecontrol signals 124, 126, the current is first "pushed" and then"pulled" through the primary winding of the transformer 54. For example,when the first transistor 116 is ON, the second transistor 118 is OFF,and current flows through the first transistor 116 and a voltage isinduced in the secondary winding of the transformer 54. Conversely, whenthe second transistor 118 is ON, the first transistor 116 is OFF, andcurrent flows in the opposite direction through the second transistor118 and the primary winding of the transformer 54. The push-pullarrangement according to the present invention reduces the magnetizationof the core of the high voltage transformer which would occur if theprimary winding was excited in only one direction, e.g. CLASS A mode.Because the operation of the transistors 116, 118 alternates the currentdirection in the primary winding, the magnetic field in the transformercore is allowed to collapse during the time lag between respectivepulses in the signals 124, 126 (FIG. 5). This allows the transformer 54to operate more efficiently. As shown in FIG. 3(a), a resistor 430 and acapacitor 431 is connected across the primary winding of the transformer54. The resistor 430 and the capacitor 431 help control transients whichmay arise in the primary winding as a result of the switching of thetransistors 116, 118.

Referring again to FIG. 3(a), each time one of the transistors 116, 118is switched on, a current flows in the primary winding and a voltage isinduced in the secondary winding of the transformer 54. The secondarywinding of the transformer 54 "steps-up" induced voltage and the inducedvoltage is increased up to 135 kV through the operation of the highvoltage multiplier 56.

According to another aspect of the present invention, the high voltagetransformer 54 comprises a multi-core arrangement as depicted in FIGS.6(a) and 6(b). The transformer 54 comprises a primary winding 55, asecondary winding 57, and a multiple core 59. The multiple core 59comprises three ferrite cores 61,63,65. Each ferrite core 61,63,65 hasair gaps 67,69 to reduce hysteresis in the core. The primary winding 55comprises 18 turns for each core 61,63,65, and the turns ratio for thesecondary winding 57 is approximately 1/50. The secondary winding 57 ispreferably vacuum sealed in epoxy resin. It will be appreciated that themultiple core arrangement depicted in FIG. 6 has the advantage oflimiting the Eddy currents to each core 61,63,65.

According to this aspect of the invention, the transformer 54 comprisesa triple core arrangement 59 in order to generate a high voltage outputon the secondary 57 without requiring a high turns ratio. By limitingthe turns ratio, the size of the transformer 54 is reduced.

The triple core arrangement for the transformer 54 shown in FIG. 6 issuitable for a 150 Watt system. For a 100 Watt system, a two corearrangement, e.g. 61 and 63, is possible with the turns ratio for thesecondary winding suitably modified. For a 50 Watt system, a single corearrangement is possible with the turns ratio suitably modified.

To further increase the output to 135 kV, the transformer 54 is operatedin resonance. The transformer 54 and the high voltage multiplier 56comprise a "tuned" circuit. The higher frequency the more efficient thetransformer 54 and the higher the output, however, the limitationbecomes the switching frequency of the transistors 116, 118 in the pulsecontrol module 50. For the transistors 116, 118 being used utilized theswitching frequency is selected in the range 16 kHz to 22 kHz. The airgap 67,69 is approximately 0.004 inches and adjusted so that thetransformer 54 produces 7.5 kV peak output at resonance.

As shown in FIG. 3(a) the secondary winding is coupled to the highvoltage multiplier 56. The high voltage multiplier 56 comprises a seriesof cascaded stages. The multiplier comprises seven cascaded stages,three of which are shown and denoted by references 136, 138, 140. Eachcascaded stage is formed from a series of capacitors and diodes. Thecapacitors and diodes are configured as a voltage multiplier as will beunderstood by those skilled in the art. (If the insulated wire 23 isused with the ring 30, the capacitance of the wire 23 is factored intothe cascade stage.) The function of the high voltage multiplier 56 is tofurther increase or multiply the "step-up" voltage produced in thesecondary winding of the transformer 54 (through the "push-pull"switching of the 140 VDC in the primary winding of the transformer 54).

In the present configuration, i.e. the tuned circuit comprising thetransformer 54 and the voltage multiplier 56, the output is as high as135 kV, and the capacitors are 680 picofarads and rated at 15 KV. Thediodes are rated for 35 kV RMS at 2 milliamperes (mA). The output of thehigh voltage multiplier 54 is electrically coupled to the discharge rod20 by the electrical contact 21 (FIG. 1). The "tuned" high voltagemultiplier 56 is shown in FIG. 7 and described below. With suitablemodifications to the high voltage transformer 54 and the high voltagemultiplier 56 (e.g. increasing the number of stages), an output voltagearound 180 kV may be achieved.

Referring to FIG. 3(a), a third winding 142 on the core of the highvoltage transformer 54 provides an input 144 for the output regulator64. The function of the output regulator 64 is to regulate or controlthe output voltage produced by the circuit 14. The regulator 64 is alsoconfigurable to regulate the output current. In voltage regulation mode,the output voltage level is held at a level as set through thepotentiometer 146 (FIG. 3(c)). In current regulation mode, the currentis maintained at a predetermined level, e.g. 250 μA and the voltage isallowed to vary between 60 kV to 135 kV, and will depend on the currentneeded to charge the particulate in the air 34.

The output voltage is regulated by controlling the duty cycle of thepulses 124, 126 (FIG. 3(b)) based on the desired output voltage level.The output voltage level is set by the potentiometer 146 shown in FIG.3(c), which is coupled to the winding 142. The potentiometer 146 ispreferably located inside the control panel 38 so as to be accessibleonly to a trained technician. The wiper of the potentiometer 146 formsthe terminal 23 and is coupled to a Zener diode 148 at terminal 155 inthe regulator 64. The Zener diode 148 provides the input for theregulator 64. The other terminal 21 of the adjust dial 42 is connectedto the terminal 144 at the winding 142.

Referring to FIG. 3(b), the output regulator 64 comprises a NPNtransistor 150 and a PNP transistor 152. The cathode of the Zener diode148 is coupled to the collector of the NPN transistor 150 through aresistor 432 and the anode of the diode 148 is coupled to the base ofthe NPN transistor 150 through a resistor 433. The base of the NPN 150is also coupled to signal ground through a diode 434. The collector ofthe NPN 150 is coupled to the base of the PNP transistor 152 through aresistor 435. The base of the PNP 152 is also coupled through a resistor436 to the output 154 from the overload protection module 66. Theemitter of the PNP 152 is tied to 12 Volts through two diodes 437. Thediodes 437 bias the PNP 152 so that the minimum pulse width is limitedto 0.2 μs. The collector of the PNP transistor 152 provides the outputfor the regulator 64 and is coupled to the pulse control module 50through a capacitor 438. The resistor 435 and capacitor 438 produce abias voltage for the resistor/capacitor networks for the 555 timers 132,138 and control the duty cycle of the respective pulse signals 124, 126.

In voltage regulation mode, the voltage induced in the winding 142 (FIG.3(a)) is proportional to the output voltage at the output 58 of thecircuit 14. When the voltage in the winding 142 exceeds the thresholdlevel (as set by the potentiometer 146--FIG. 3(c)), the zener diode 148will conduct causing the NPN transistor 150 to turn ON. This in turncauses the PNP transistor 152 to turn ON and the bias voltage on thecapacitor 438 changes thereby causing the pulse width and the duty cycleof the pulse signals 124, 126 to decrease. By varying the duty cycle ofthe signals 124, 126, the level at the high voltage output 58 andionization is varied. In the present embodiment, the output voltage isregulated to 135 kV at 1.0 mA, or to 100 kV at 1.5 mA.

Referring to FIG. 3(b), when the overload protection module 66 istriggered the output 154 is pulled LOW and the PNP transistor 152 isturned OFF. This effectively disables the 555 timers 132, 134 as will bedescribed in more detail below.

In another aspect, the output regulator 64 allows the output currentlevel to be controlled. It has been found that current regulation isideal for cleaning air which contains a lot of particulate matter, andthe more particulate matter the easier it is to charge and maintain thecharge. In other words, the more particulate matter the lower thecurrent required once the particulate is charged. Voltage regulation, onthe other hand, is preferable when the air is relatively clean, e.g.office spaces, because it takes more current to charge the particulatein current regulation mode.

To select current regulation, there is a terminal 151 which is connectedto node 170 (i.e. the output of the filter network 87) and a switch orjumper 153. The jumper 153 couples the terminal 151, i.e. output of thefilter 87 to the anode of the zener diode 148 at terminal 155. Incurrent regulation mode the NPN transistor 150 is coupled to the outputof the filter network 87, and the current flowing controls the NPNtransistor 150 which in turn controls the PNP transistor 152 and thebias voltage on the capacitor 438. For the component values shown in theTable below, the output current is regulated at 250 μA.

As shown in FIG. 2, the filter network 87 couples the output signal fromthe high voltage multiplier 56 to the overload protection module 66. Thefunction of the filter network 87 is to condition the output signal fromthe voltage multiplier 56 in order to prevent false triggering of theprotection module 66.

Referring to FIG. 3(b), the filter network 87 comprises a branch havingresistors 445 to 448 and capacitors 449 to 451 connected as shown. Theother branch of the filter network 87 comprises capacitor 452. Thefilter 87 has an input terminal 162 which is connected to the secondarywinding of the high voltage transformer 54. The frequency characteristicof the filter network 87 is configured according to the resonantfrequency of the transformer 54. The exemplary values for the resistorsand capacitors provided in the Table are suitable for the 19 kHzswitching frequency. For 16 kHz operation, a suitable value for theresistors 445 to 448 is 1.1 K, and for 20 kHz operation, a suitablevalue for the resistors 445 to 448 is 560 Ohms.

As shown in FIG. 3(a), the output 164 from the high voltage multiplier56 includes a network 166 comprising a varistor 168, a blocking diode452, resistors 453 and 454, and capacitors 455, 456 connected as shown.The function of the network 166 is to "smooth" output signal tapped fromthe voltage multiplier 56. The varistor 168 absorbs spikes in the outputsignal from the multiplier 56 by shunting them to ground before they canreach the overload protection module 66.

Referring back to FIG. 3(b), at the input terminal 162 to the filter 87,the signal is split into the two branches. One branch shifts the phaseof the signal forward, while the other branch shifts the phase of thesignal back, so that when the signal is recombined at node 170, i.e. theinput to the overload protection module 66, the ripple in the signal iseffectively cancelled. In current regulation mode, the signal from thefilter network 87 provides the input to the zener diode 148 as describedabove.

Referring to FIG. 3(b), the signal from the filter network 87 is inputto the overload protection module 66 at node 170. The overloadprotection module 66 comprises first and second thyristors or siliconcontrolled rectifiers (SCR) 172 and 174. The first SCR 172 providesprotection for overload voltage conditions. The second SCR 174 providesprotection for temperature overload conditions. The SCR 172 disables thecircuit 14 if a predetermined output current level is exceeded. Thesecond SCR 174, on the other hand, disables the circuit 14 if a safeoperating temperature is exceeded, for example 75° C. As shown in FIG.3(b), each SCR 172, 174 includes an input network denoted respectivelyby 173 and 174.

The gate of the first SCR 172 receives the output signal from the filternetwork 87 (i.e. node 170) through a resistor 457. The value of theresistor 457 is selected so that the SCR 172 is triggered at theappropriate output level. (As described, the filter network 87 removesthe ripples or spikes in the signal to prevent false triggering of theSCR 172.) The gate of the SCR 172 includes a diode 458 and capacitor 459which form an input 176 for connecting to terminal 94, i.e. the outputthe short circuit protection module 67 shown in FIG. 3(a). The gate ofthe SCR 172 is also connected to signal ground through a thermistor 178and a resistor 460 as shown in FIG. 3(b). The function of the thermistor178 is to compensate the SCR 172 when the unit becomes warm. As thetemperature rises inside the enclosure 12, the SCR 172 will become moresensitive and susceptible to false triggering.

In operation, when the output current exceeds the predeterminedthreshold level, the SCR 172 is triggered and the output of the SCR 172goes LOW. (As shown in FIG. 3(b), the output of the SCR 172 is tied to+12 Volts through resistor 461.) When the output of the SCR 172 goesLOW, the pulse signals 112, 114 to the respective monostable vibrators128, 130 are disabled, which, in turn, prevents the power transistors116, 118 (FIG. 3(a)) from switching. As shown in FIG. 3(b), the outputof the SCR 172 is also coupled to the base of the PNP transistor 152 inthe regulator 64. Triggering of the SCR 172 also causes the regulatorinput to the 555 timers 132, 134 to be disabled. The monostablevibrators or pulse generators 128, 130 remain in the disabled stateuntil the SCR 172 is reset by the reset module 68.

As shown in FIG. 3(b), the reset module 68 comprises a timer 180 and anoutput transistor 182. The timer 180 is configured to produce an outputsignal which turns on the transistor 182 after a predetermined time. Theoutput transistor 182 is connected across the SCR 172. When turned on,the transistor 172 effectively "shorts-out" the SCR 172 and the SCR 172is reset. (The SCR 172 resets below 0.7 Volts and the collector-emittervoltage for the transistor 182 in saturation is 0.2 Volts.) The timer180 is implemented using the MC14566 industrial time base generator chipavailable from Motorola Corporation. The timer chip 180 has an input 184which is connected to the output of the timer (oscillator) chip 96 forreceiving a timing signal 186. The timer 180 is configured to produce anoutput signal for turning on the transistor 182 approximately every 4minutes (PIN 12 of the oscillator chip 96 is tied to +12 Volts) orapproximately every 4 seconds (PIN 12 of the oscillator chip 96 tied toGROUND) as will be within the understanding of one skilled in the art.As shown in FIG. 3(b), the output of the timer 180 is coupled to thebase of the transistor 182 through resistor 462. The timer 180 itself isreset through the operation of the transistor 182. As shown, the timer180 has a reset input 188 which is coupled to the collector of thetransistor 182 through a resistor 463 and a capacitor 464.

Referring to FIG. 3(b), the second SCR 174 in conjunction with athermistor 190 provides the over-heating protection for the high voltagecircuit 14. The thermistor 190 is coupled to the gate of the SCR 174through a zener diode 192, a resistor 465, and another zener diode 194as shown in FIG. 3(b). The output of the SCR 174 is tied to +12 Voltsthrough resistor 466 and also to the buffer inputs 104, 108 to themonostable vibrators 128, 130. When the operating temperature exceeds apredetermined threshold, e.g. 75° C., the SCR 174 is triggered and pullsthe input to the buffers 104, 108 to ground thereby disabling themonostable vibrators 128, 130 (i.e. pulse generators). Since a hightemperature condition may indicative of a malfunction, as opposed to anoverload condition, a blocking diode 196 coupled to the SCR 174 preventsthe SCR 174 from being automatically reset by the reset module 180. Toreset the SCR 174 the unit must be powered down which is appropriatelydone by a technician who will also inspect the for a malfunction.

The overload protection module 66 also includes terminals 198, 200 forconnecting to the current meter 48 (FIG. 3(c)). The terminal 198 isformed at the junction of capacitor 467 and resistor 468 which isconnected to the output of the filter network 87 at node 170. Theterminal 200 is formed at the junction of resistor 469 and signalground. The other terminal of resistor 469 is connected to the gate ofthe SCR 174. The terminal 198 is connected to terminal 27 of the meter48 and terminal 200 is connected to the "return" terminal 25 as shown inFIG. 3(c). The meter 48 includes a calibration resistor 202. Terminal 25of the meter 48 is connected to the potentiometer 146 through resistor470 as shown in FIG. 3(c).

As shown in FIG. 3(a), the power output indicator 46 is also connectedto the winding 142 through a drive circuit 158. The lamp 46 is connectedto the drive circuit 158 at terminal 19 (FIG. 3(a)). The drive circuit158 for the lamp 46 comprises a transistor 160. The base of thetransistor 160 is coupled to the winding 142 through resistor 439,capacitor 440 and rectifying diode 441. The collector of the transistor160 is coupled to terminal 19 through diode 442. The drive circuit 158also includes another diode 443 and resistor 444 which couple theterminal 19 to signal ground.

In operation, the lamp 46 will glow dimly when the unit is on. When avoltage is induced in the winding 142, a base current will flow causingthe transistor 160 to turn ON and the collector current causes the lamp46 to glow brightly. If the output of the ionizer 4 has been shorted,the continuous resetting of the overload protection module 66 will causethe lamp 46 to flicker every 4 minutes (or 4 seconds).

Referring to FIG. 3(a), the anode of the diode 441 is also coupled to aterminal 220. The terminal 220 connects to the input terminal of acircuit 222 shown in FIG. 3(b). The circuit. 222 provides outputterminal 224 and test point terminal 226. The circuit 222 processes thenegative portion of the output at the coil 142. The circuit 222comprises capacitors 471,472, diode 473, and resistor 474. Exemplarycomponent values are provided in the Table below.

As described above, the high voltage multiplier 56 and the transformer54 can comprise a "tuned" circuit. Reference is next made to FIGS. 7(a)to 7(c) which show the high voltage multiplier 56 according to thisaspect of the present invention. The high voltage multiplier 56comprises an enclosure denoted generally by reference 204. The enclosure204 comprises a compartment 206 for housing the cascaded stages 136,138, 140 (FIG. 3(a)) of the multiplier 56 and a tube 208 for connectingthe output wire 58. The cascaded stages (e.g 136, 138, 140) are formedfrom diodes and capacitors denoted respectively by D and C. The input tothe multiplier 56 is connected to the secondary winding of the highvoltage transformer 54 through an AC wire and a ground wire. A principlefunction of the high voltage multiplier 56 is to lower the capacitancebetween the AC and ground in order to operate the transformer 54 inresonance.

As shown in FIG. 7, the compartment 206 has respective side channels210, 212 for mounting the capacitors C and a bottom channel 214 formounting the diodes D. The channels 210, 212, 214 are preferably filledwith an epoxy material. The enclosure 204 includes one or more channels216 for receiving dielectric materials in order to change thecapacitance and therefore the impedance and output voltage produced bythe high voltage multiplier 56.

For a 135 kV output, the high voltage multiplier 56 comprises 9 stages,and the capacitors C are 680 pF and rated at 15 kV, and the diodes D arerated at 35 kV and 2 mA.

It will be appreciated that the high voltage circuit 14 according to thepresent invention provides an elegant and cost-effective solution toimplementing the ionizer 4. The high voltage circuit 14 combined withthe modular design of the ionizer 4 provides a device which can easilybe integrated with the existing duct work or arranged as an ionizer bankto replace known mechanical filter banks in an office building forexample.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof.Therefore, the presently discussed embodiments are considered to beillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are therefore intended to be embraced therein.

    ______________________________________                                        TABLE FOR COMPONENT VALUES                                                    ______________________________________                                        Varistor       41      120 V                                                  Potentiometer 146      50K                                                    Zener Diode   148      15 V                                                   Transistor    150      4624                                                   Transistor    152      4626                                                   Transistor    160      U45                                                    Varistor      168      20 V                                                   SCR 1         172      5061                                                   SCR 2         174      5061                                                   Thermistor    178      100K, 20° C.                                    Transistor    182      4124                                                   Thermistor    190      100K, 20° C.                                    Zener Diode   192      7.8 V                                                  Zener Diode   194      7.5 V                                                  Diode         196      914                                                    Resistor      400      1.5, 10 W                                              Diode         401      8 A, 400 V                                             Capacitor     402      680 μF, 200 V                                       Resistor      403      100K, 1 W                                              Diode         404      4007                                                   Capacitor     405      1000 μF, 40 V                                       Capacitor     406      2.2 μF                                              Capacitor     407      2.2 μF                                              Resistor      408      100K                                                   Resistor      409      220K                                                   Capacitor     410      1 nF                                                   Resistor      411      100K                                                   Resistor      412      220K                                                   Capacitor     413      180 pF                                                 Capacitor     414      50 pF                                                  Diode         415      914                                                    Diode         416      914                                                    Resistor      417      18K                                                    Resistor      418      18K                                                    Resistor      419      18K                                                    Resistor      420      18K                                                    Resistor      421      6.8K                                                   Resistor      422      1.8K                                                   Resistor      423      18K                                                    Resistor      424      18K                                                    Resistor      425      6.8K                                                   Capacitor     426      1 nF                                                   Capacitor     427      1 nF                                                   Resistor      428      27 Ohms                                                Resistor      429      27 Ohms                                                Resistor      430      5.6K                                                   Capacitor     431      150 pF                                                 Resistor      432      18K                                                    Resistor      433      820K                                                   Diode         434      914                                                    Resistor      435      18K                                                    Resistor      436      47K                                                    Diodes        437      4126                                                   Capacitor     438      0.1 μF                                              Resistor      439      10K                                                    Capacitor     440      2.2 μF, 100 V                                       Diode         441      4007                                                   Diode         442      4007                                                   Diode         443      4007                                                   Resistor      444      100 Ohms, 1 W                                          Resistor      445      680 (1.1K, 560 Ohms)                                   Resistor      446      680 (1.1K, 560 Ohms)                                   Resistor      447      680 (1.1K, 560 Ohms)                                   Resistor      448      680 (1.1K, 560 Ohms)                                   Capacitor     449      15 nF                                                  Capacitor     450      15 nF                                                  Capacitor     451      15 nF                                                  Capacitor     452      1 nF                                                   Resistor      453      18 Ohms                                                Resistor      454      1K, 1 W                                                Capacitor     455      0.1 μF                                              Capacitor     456      2.2 μF, 100 V                                       Resistor      457      220K                                                   Diode         458      914                                                    Capacitor     459      1 nF                                                   Resistor      460      57K                                                    Resistor      461      1.5K                                                   Resistor      462      18K                                                    Resistor      463      1M                                                     Capacitor     464      47 pF                                                  Resistor      465      18K                                                    Resistor      466      18K                                                    Capacitor     467      2.2 μF                                              Resistor      468      2.2K                                                   Resistor      469      220K                                                   Resistor      470      12K                                                    Capacitor     471      0.1 μF                                              Capacitor     472      680 pF                                                 Diode         473      4007                                                   Resistor      474      6.8K                                                   Resistor      475      5K                                                     Capacitor     476      2.2 μF                                              ______________________________________                                    

I claim:
 1. An apparatus for purifying gas flowing in a duct byestablishing a radially directed ionic wind within the duct to sweepparticulate solids directly onto one or more collector electrodes, saidapparatus comprising:(a) an ionizing unit; (b) means for supporting saidionizing unit within the duct, said ionizing unit comprising,(i) awater-tight housing, (ii) a high voltage generator within the housingfor generating a high voltage output, said high voltage generatorincluding a transformer and control means for energizing saidtransformer to produce said high voltage output, said control meanshaving pulse generator means for generating pulses and a push-pull drivecircuit coupled to said transformer and responsive to said pulses forenergizing said transformer, and said high voltage generator includingvoltage multiplier means coupled to the output of said transformer formultiplying the output of said transformer to said high voltage output,and regulator means for regulating output current, (iii) an electrodesupport rod coupled to said high voltage output and extending from saidhousing coaxially within said duct, iv) at least one group of ionizingelectrodes mounted on said support rod and extending radially therefrom;and (c) means for connecting said high voltage generator to an externallow voltage power supply.
 2. The apparatus as claimed in claim 1,wherein the duct wall comprises a collector electrode for collectingsaid particulate solids.
 3. The apparatus as claimed in claim 2, whereinsaid ionizing unit includes a plurality of axially spaced groups ofionizing electrodes.
 4. The apparatus as claimed in claim 1, furtherincluding a ring electrode positioned inside the duct and surroundingone group of said ionizing electrodes, said ring electrode having adiameter spanning a portion of the duct and being electrically connectedto ground.
 5. The apparatus as claimed in claim 4, wherein said ringelectrode is supported by said electrode support rod.
 6. The apparatusas claimed in claim 4, wherein said ring electrode is connected to andsupported by the duct.
 7. The apparatus as claimed in claim 1, whereinsaid regulator means comprises means for regulating said high voltageoutput.
 8. The apparatus as claimed in claim 1, wherein said ionizingunit includes overload protection means for disabling said high voltagegenerator when said high voltage current output exceeds a predeterminedlevel.
 9. An air purifier for purifying air in an enclosed space in abuilding and said enclosed space being provided with an AC power supply,said air purifier comprising:(a) an enclosure having at least onecollecting electrode; (b) an ionizing unit; (c) means for supportingsaid ionizing unit inside said enclosure, said ionizing unitcomprising,(i) a water-tight housing, (ii) a high voltage generatorwithin said housing for generating a high voltage output, said highvoltage generator including a transformer and control means forenergizing said transformer to produce said high voltage output, saidcontrol means having pulse generator means for generating pulses and apush-pull drive circuit coupled to said transformer and responsive tosaid pulses for energizing said transformer, and said high voltagegenerator including voltage multiplier means coupled to the output ofsaid transformer for multiplying the output of said transformer to saidhigh voltage output, and regulator means for regulating output current,(iii) an electrode support rod coupled to said high voltage output andextending from said housing coaxially within said duct, (iv) at leastone group of ionizing electrodes mounted on said support rod andextending radially therefrom for establishing a radially directed ionicwind within said enclosure to sweep particulate solids in the airdirectly onto said collector electrode; (d) means for connecting saidhigh voltage generator to the external AC power supply; and saidenclosure including an air intake port and an air exhaust port.
 10. Theapparatus as claimed in claim 9, further including a fan for pulling airthrough said intake port and out through said exhaust port.